<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        .search {
            position: relative;
            width: 178px;
            margin: 100px;
        }
        .con {
            display: none;
            position: absolute;
            top: -40px;
            width: 171px;
            border: 1px solid rgba(0, 0, 0, .2);
            box-shadow: 0 2px 4px rgba(0, 0, 0, .2);
            padding: 5px 0;
            font-size: 18px;
            line-height: 20px;
            color: #333;
        }
        .con::before {
            content: '';
            width: 0;
            height: 0;
            position: absolute;
            top: 28px;
            left: 18px;
            border: 8px solid #000;
            border-style: solid dashed dashed;
            border-color: #fff transparent transparent;
        }
    </style>

</head>
<body>
    <div class="search">
        <div class="con">123</div>
        <input type="text" placeholder="请输入您的快递单号" class="jd">
    </div>
    <script>
        var con = document.querySelector('.con');
        var jd_input = document.querySelector('.jd');
        jd_input.addEventListener('keyup', function(){
           if(this.value == ''){
            con.style.display = 'block';
           }else{
            con.style.display = 'block';
            con.innerText = this.value;
           }
        })
        //当我们失去焦点，就隐藏这个con盒子
        jd_input.addEventListener('blur', function(){
            con.style.display = 'none';
        });
        jd_input.addEventListener('focus', function(){
            if(this.value !== ''){
            con.style.display = 'block';
        }
        });
    </script>
</body>
</html>